1. Field of the Invention
The present invention relates to a time-division multiplex communication system having a transmission path which links or connects a plurality of independent signal data links.
2. Description of the Prior Art
Heretofore, time-division multiplex communication systems generally have a time-divided highway composed of time slots each fixedly assigned to a certain channel or link. Even if the time slots are of a multiframe structure, each of the channels is independent per frame, and has a constant transmission period or transmission rate.
Japanese Patent Laid Open No. 220848/92 discloses a time-division multiplex communication system in which a plurality of time slots are variably combined into various bandwidths for providing extended channels of variable rates. Since there is a limitation on the number of time slots in one frame, if more time slots are to be introduced for providing an extended channel, then it has heretofore been necessary to move or delete another channel in which such time slots have been used, or to reserve a maximum number of time slots corresponding to a required bandwidth for the extended channel.
For multilink operation sharing the time slots over a link transmission path, it has been necessary to terminate the individual links at the inlet and outlet ends of the link transmission path and multiplex the link transmission path using a multilink protocol.
In the conventional time-division multiplex communication systems described above, the transmission rate of a time slot assigned to each channel is constant at all times irrespective of the amount of signals flowing through the time slot. Therefore, if the conventional time-division multiplex communication systems incorporate independent links for which the average amount of signals is relatively small and the loads vary, then since each of the channels is required to be associated with a transmission path necessary to carry the largest traffic load, the efficiency of the system is usually low.
In the case where a plurality of time slots are variably combined depending on the load to provide extended channels of variable rates, because the number of time slots in one frame is predetermined, if more time slots are to be introduced for providing an extended channel, then other channels may be affected as another channel in which such time slots have been used has to be moved or deleted. If a maximum number of time slots corresponding to a required bandwidth are assigned beforehand to an extended channel in order that other channels may not be affected, then the assigned time slots are not effectively used in usual times at which the load is small, resulting in idle slot times which lower the efficiency of the time-divided highway.
If a multilink arrangement which shares certain time slots is employed according to a multilink protocol in order to absorb load variations of the links, then since the individual links have to be terminated at the inlet and outlet ends of the link transmission path, the penetrability of the multilink protocol between the terminated ends of the links cannot be assured.